The invention concerns a voltage converter comprising a transformer through which a primary circuit is connected to at least one secondary circuit, wherein a voltage applied to the input of the primary circuit is converted to output voltages on respective secondary circuits.
A.H. Weinberg and L. Ghislanzoni: "A new Zero Voltage and Zero Current Power Switching Technique." PESC 89 provides a DC voltage converter with a constant ratio of conversion. The converter uses a transformer through which a primary circuit is coupled to one or more secondary circuits. The primary circuit has two electrical switches which periodically reverse the voltage across the primary windings wound around the core of the transformer. When the first switch is switched off, double voltage will be present across the second switch. The parallel inductance of the transformer now forms a resonance circuit with the parasitic capacities across the switches. The magnetizing current in the transformer resonantly recharges these capacities so that there will be no voltage across the second switch after a period of time. The second switch can then be switched on without instantaneous great voltage changes and with a greatly reduced loss. When the second switch is switched off again, a corresponding process takes place. This art is also described in the European Patent Specification EP 0 077 958.
When one of the switches is switched on, a series resonance circuit will be formed, consisting of an inductance in series with the transformer (the inductance may e.g. consist of the parasitic series inductance of the transformer) as well as a capacity on either the primary side or on each of the outputs on the secondary side, and resonant energy transfer takes place from the primary side to the secondary side. When the current in the resonance circuit is zero, the on-switch can be switched off without instantaneous great current changes and with a greatly reduced loss.
The prior art operates satisfactorily as long as the loads on the respective secondary circuits are of the same order and are fairly constant. When the load distribution is very skew, e.g. a 90% load on one tap and a 10% load on another tap, the output voltage drops in relation to the nominal value on the tap most loaded. Correspondingly, the voltage rises with respect to the nominal value on the tap least loaded. These deviations from the nominal voltage values (cross-regulation) entail that the voltage converters of the prior art are not useful where precise feed voltages are required, and where extreme load distributions may occur.
Further, it is necessary that the employed transformer is optimized with respect to the parasitic series inductance such that in case of capacitive tuning on the primary side the parasitic series inductance on the secondary side must be insignificant and reverse. If this requirement is not satisfied, the tuning frequency varies with the load distribution, and the switches can no longer be expected to be switched off with low losses. It may be difficult to fully satisfy this requirement in practice, and even partial satisfaction of the requirement may be inconsistent with the requirement of low power losses in the transformer.